TiO2-graphene-silver hybrid nanocomposite and a method of preparation thereof

ABSTRACT

A TiO2-graphene-silver hybrid nanocomposite and a method of preparing the TiO2-graphene-silver hybrid nanocomposite is disclosed. The TiO2-graphene-silver hybrid nanocomposite at an average particle size ranging from 12-15 nanometers and having a surface area of 140.5 m2/g includes titanium oxide, graphene oxide and silver, the silver ranging from about 2 weight % to 10 weight %. The method of preparation includes introducing sol gel to a microwave irradiation to prepare an irradiated sample of TiO2-graphene oxide sample, wherein the sol gel includes TiO2 containing gel along with graphene containing sol, followed by adding AgNO3 solution to the TiO2-graphene oxide sample for preparing a TiO2-graphene-silver hybrid suspension. The TiO2-graphene-silver hybrid suspension undergoes microwave irradiation to prepare dried TiO2-graphene-silver hybrid composite.

FIELD OF INVENTION

The present invention generally relates to the field of hybrid nanocomposites for energy storage applications. More specifically, the present invention relates to TiO₂-graphene-silver hybrid nanocomposite employed in a supercapacitor among many energy storage applications and a method of preparation thereof.

BACKGROUND OF INVENTION

Drastic climatic changes and urbanization has shifted focus to primarily energy needs, especially in the scientific and industrial communities. The erstwhile non-renewable energy resources contribute to environment pollution, increasing mining cost, depletion of fossil fuel, etc. While titanium oxide (TiO₂) is being explored and researched as a promising candidate for supercapacitors, especially in terms of energy storage applications, owing to its characteristics like low cost, natural abundance, renewable in nature, etc. to name a few, optimum efficiency levels had not been met with mere titanium oxide (TiO₂).

Therefore, there exists a need for improvised and economic renewable energy sources with enhanced energy density, wherein enhanced energy performance may be characterized by voltage limit, specific capacitance, energy density, power density, life time and likewise.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures wherein like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.

FIG. 1a is illustrative of adsorption-desorption isotherms of TiO₂ particles using surface area analyzer (physical adsorption-desorption of N₂ at liquid-N₂ temperature) exhibiting a typical IV hysteresis loop.

FIG. 1b is illustrative of adsorption-desorption isotherms of TiO₂-graphene-silver, wherein the percentage weight of silver in the hybrid nanocomposite is 10%.

FIG. 2a is illustrative of XRD of TiO₂-graphene-silver prepared via the sol-gel synthesis technique followed by the microwave irradiation technique representing the anatase, and sliver phases, wherein the percentage weight of silver in the hybrid nanocomposite is 10%.

FIG. 2b is illustrative of the UV-vis absorption spectra of TiO₂-graphene-silver, wherein the percentage weight of silver in the hybrid nanocomposite is 10%.

FIG. 2c is illustrative of the SEM image of TiO₂-graphene-silver, wherein the percentage weight of silver in the hybrid nanocomposite is 10%.

FIG. 2d is illustrative of the photo degradation of methylene blue (MB) by the TiO₂-graphene-silver sample prepared by the microwave irradiation method, wherein the percentage weight of silver in the hybrid nanocomposite is 10%.

FIG. 3 is illustrative of cyclic voltammogram of TiO₂-graphene-silver nanocomposites at different scan rates ranging from 20 to 500 mV/s.

FIG. 4 is illustrative of the galvanostatic charge/discharge curve of TiO₂-graphene-silver nanocomposites at 100 mV/s, when the nanocomposite is employed in a supercapacitor.

FIG. 5 is illustrative of the variation of the specific capacitance of TiO₂-graphene-silver nanocomposites electrode as a function of cycle number measured at 100 mV/s.

FIG. 6 is illustrative of electrochemical impedance spectra of TiO₂-graphene-silver nanocomposites from 100 kHz to 1 Hz in the form of Nyquist plots.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in a TiO₂-graphene-silver hybrid nanocomposite and method of preparation thereof. Accordingly, the method steps and nanocomposite components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present application so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

In this document, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of objects may include not only those objects but also include other objects not expressly listed or inherent to such process, method, article, or apparatus. An object proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical objects in the process, method, article, or apparatus that comprises the object.

Embodiments of the present invention provide a TiO₂-graphene-silver hybrid nanocomposite and a method of preparation thereof.

In accordance with the present invention, a TiO₂-graphene-silver hybrid nanocomposite includes titanium oxide ranging from about 70 weight % to 98 weight %, graphene oxide ranging from about 0.1 weight % to 20 weight %, and silver ranging from about 2 weight % to 10 weight %, wherein the silver is formed on the titanium oxide. Accordingly, TiO₂-graphene-silver hybrid nanocomposite comprise essentially spherical particles with an average particle size of 12-15 nm. The tetragonal TiO₂ of the TiO2-graphene-silver hybrid nanocomposite predominantly exists in an anatase phase, silver predominantly in cubic phase and graphene oxide in characteristics phase.

Referring to FIG. 1b , a BET surface area analysis represented a surface area of 140.5 m²/g for the TiO₂-graphene-Ag nanocomposite, significantly higher than the surface area observed for TiO₂ particles at 110.8 m²/g. The increased surface area is characteristic of the synergistic effect of silver and graphene in combination, depicted by the mesopores of different pore sizes in the nitrogen physisorption measurements. Hereinafter, a method of preparing the TiO₂-graphene-silver hybrid nanocomposite will be further described.

To begin the method of preparation, pure sol of titanium oxide along with graphene containing sol is introduced to a microwave radiation of 50% microwave power, preferably at 2.45 GHz and irradiated for 18 minutes, to prepare a TiO₂-graphene oxide sample. The preparation of pure sol of titanium oxide along with graphene containing sol to initiate the preparation of the hybrid nano-composite includes a sol-gel synthesis process, wherein the sol-gel includes graphene added to the CTAB before adding TiCl₄, for example, thereby representing a combined sol-gel precursor. After irradiating the sample of TiO₂-graphene oxide, the method further includes washing the sample with ethanol and distilled water and is eventually dried for a period of 10 hours.

Accordingly, the method further includes suspending TiO₂-graphene oxide sample from the previous step, in deionized water at room temperature. A 0.5 M AgNO₃ solution is further added to the resulting suspension to prepare silver as an integral part of the resulting suspension. The amount of 0.5 M AgNO3 solution added to the resulting suspension, may vary, wherein the amount is selected from, 0.7, 1.77 and 3.55 ml to prepare 2%, 5% and 10% of silver respectively and thereby generate a TiO₂-graphene-silver hybrid suspension. The TiO₂-graphene-silver hybrid suspension undergoes reduction with 100 μl of hydrazine hydrate 80%, to be further irradiated with microwave irradiation preferably at an oscillation frequency of 2.45 GHz, for about 12-20 minutes. Thereafter, the irradiated TiO₂-graphene-silver hybrid suspension is washed with ethanol and distilled water and further dried at 80° C. overnight to generate dried and irradiated.

The dried irradiated TiO₂-graphene-silver hybrid further undergoes calcination by employing a furnace at 300° C. to produce TiO₂-graphene-silver hybrid nanocomposite wherein temperature is increased at a rate of 1° C./minute and maintained at such temperature for a time period of 3-5 hours.

In another embodiment, the sol gel synthesis of pure sol of titanium oxide along with graphene containing sol, as a combined sol, includes numerous steps. According to an example of the present embodiment, the sol gel synthesis includes preparing graphene oxide by a modified Hummer's method, wherein 4.5 g of graphite is added to 110 ml of 98% H₂SO₄ (95-98%, Basic) containing 2.5 g NaNO₃ 99.5%. The method further includes leaving the solution in an ice bath for a time period of 20 minutes at 0° C. followed by gradually introducing KMnO₄ to the solution, maintaining the temperature at 0° C. The resulting solution is left to stand at 40° C. for two hours with vigorous stirring, wherein the color of the solution turns to dark green. Furthermore, 230 ml of deionized water is added gradually to maintain the solution temperature below 50° C. An addition of 20 ml of H₂O₂ is made to the solution and maintained as such for a time period of 20 minutes. Thereafter, the solution is centrifuged at 4000 rpm repeatedly for a time period of 10 minutes, and further washed with 10% HCl and deionized water to render the solution acid/nitrate free and finally dried at 50° C. overnight to obtain a dry brown paste of graphene oxide.

Accordingly, sol-gel synthesis of TiO₂ includes adding TiCl₄ dropwise to 0.5 g of N-Cetyl-N,N,N-Trimethyl ammonium Bromide (CTAB) to generate a pale yellow precipitate, following which a suitable amount of 1.6 M NH₄OH solution was prepared to further neutralize the acidic solution and eventually produce a pure TiO₂ sol. The pure TiO₂ sol is maintained at a temperature of 80° C. for 24 hours in a water bath to form a gel, to be further filtered and washed with absolute alcohol and distilled water. Thereafter, the gel is dried at a temperature of 80° C. for 10 hours in a drying oven.

Different samples containing different percentage of grapheme like 0.1%, 1%, 2%, 5%, 10% and 20%, require 0.019, 0.19, 0.038, 0.95, 1.9 and 3.8 g of graphene respectively to be added to the CTAB before addition of the TiCl₄, as outlined in the above steps, to prepare the combined sol for initiating microwave irradiation.

The method of preparation of TiO₂-graphene-Ag nanocomposites results in the formation of tetragonal TiO₂ predominantly in an anatase phase and silver predominantly in a cubic phase along with the characteristics phase of graphene oxide, wherein silver is about 10 weight %, as represented in FIG. 2a (XRD pattern). FIG. 2b is representative of the UV spectrum of the prepared TiO₂-graphene-Ag nanocomposites depicting the characteristic peak at 240 nm. Characterization studies represented by the SEM image as represented by FIG. 2c represents an average particle size of 12-15 nm. FIG. 2d includes photodegradation of methylene blue representing efficiency of 43% of the prepared TiO₂-graphene-silver nanocomposites over existing nanocomposites, the photodegradation occurring under the illumination of 254 nm (UV) for a duration of 180 minutes.

In another embodiment of the present invention, the TiO₂-graphene-silver hybrid nanocomposite is employed as an active material in a supercapacitor system, assembled in a two electrode system. Considering an example in accordance with the embodiment, wherein coin cells were assembled in a vacuum glove box of a supercapacitor, wherein the vacuum glove box is provided with an argon atmosphere. In line with the example, test coin cell comprises a metal cap, a metal case with polymer seal, a spring, two stainless steel collectors coated with active materials and a membrane separator accommodated in 1.2 M LiPF₆ in EC/DMC (1:1, v/v) was used as an electrolyte.

Capacitive behavior studies of supercapacitors were conducted, wherein the capactive behavior studies include cyclic voltammetry (CV), galvanostatic charge/discharge, and Electrochemical impedance spectroscopy (EIS) test on an electrochemical workstation (GAMRY-3000, USA) at room temperature. Galvanostatically charged/discharged was performed at current density of 100 mA/g in a potential range of 0 to 2.8 V. CV was carried out at various scan rates in the range from 20 mV/s to 500 mV/s scan rate, while (EIS) analyses was performed by applying a perturbation voltage of 100 mV/s in a frequency range between 1 Hz and 100 kHz. The cycling stability for the supercapacitors was tested by using a continuous charge-discharge cycling at a current density of 100 mA/g.

In reference to FIG. 3, TiO₂-graphene-silver nanocomposites of the present invention employed in energy storage applications, exhibits a quasi-rectangular shape, indicating an ideal capacitive behavior at different scan rates of 20, 30, 50, 100, 200, 300, 400 and 500 mV/s in the potential window of 0-2.8 V. Furthermore, referring to the linear increase of the current with increasing scan rate, it is indicated that the charge is primarily non-faradic in nature. In reference to FIG. 4, the galvanostatic charge/discharge curve of TiO₂-graphene-silver nanocomposites is represented at 100 mV/s, TiO₂-graphene-silver hybrid nanocomposite is employed as an active material in a supercapacitor system.

Now referring to FIG. 5, wherein variation of the specific capacitance of TiO₂-graphene-silver hybrid nanocomposites electrode as a function of cycle number measured at 100 mV/s is represented as long term cycle stability is a crucial parameter of to be studied with respect to supercapacitors. Advantageously, the present invention exhibits a very long cycle life over the entire cycle number, when evaluated by repeating the CV measurements between 0 and 2.8 V at a scan rate of 100 mV/s for 250 cycles as shown in FIG. 5. Furthermore, the specific capacitance as a function of cycle number is maintained a constant even after 250 cycles, indicative of a good cycling life of the electrode materials, wherein the electrode materials is made of the TiO₂-graphene-silver hybrid nanocomposites of the present invention. Furthermore, the specific capacitance (Cs), of the electrode is at a scan rate of 100 mV/s and current density of 100 mA/g is 54 F/g, almost 5 times higher than that of the TiO₂/Ti electrode (11.70 F/g), thereby representing excellent capacitance as opposed to the erstwhile supercapacitor.

With reference to FIG. 6, EIS measurements were carried out at the open-circuit potential in the frequency range of 1-100 kHz with an AC perturbation of 100 mV to evaluate the fundamental capacitive behavior of electrode materials when employed in supercapacitor applications. The Nyquist plots of TiO₂-graphene-silver nanocomposites is a plot depicting the frequency response of the electrode material/electrolyte system against the imaginary component (−Z″ or −Z_(imag)) of the impedance against the real component (Z′ or Z_(real)). Again referring to FIG. 5, the impedance plot is composed of a semicircle arc in the high frequency region followed by a relative linear response in the low frequency region, wherein the high frequency region corresponds to the charge transfer limiting process. Also, the diameter of the semicircle corresponding to a resistance of 24 ohm, indicates that the charge transfer resistance of the TiO₂-graphene-silver nanocomposites is low and thereby more conductive. Furthermore, the linear part in the low frequency region is related to the Warburg resistance (diffusive resistance) of the electrolyte into the interior of the electrode surface and an ion diffusion/transport into the electrode surface. A line almost vertical to the real axis in the imaginary part of the impedance at the low-frequency region, represents the swift ion diffusion in electrolyte and the adsorption onto the electrode surface thereby facilitating an ideal capacitive behavior of the electrodes, whose active materials are composed of the TiO₂-graphene-silver nanocomposites of the present invention.

In the foregoing specification, specific embodiments of the invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 

What is claimed is:
 1. A method of preparing a TiO₂-graphene-silver hybrid nanocomposite, the method comprising: introducing sol gel to a microwave irradiation at 50% microwave power for a time period of 18 minutes to prepare an irradiated sample of TiO₂-graphene oxide sample, wherein the sol gel includes TiO₂ containing gel along with graphene containing sol; adding 0.5 M AgNO₃ solution to the TiO₂-graphene oxide sample for preparing a TiO₂-graphene-silver hybrid suspension; irradiating the TiO₂-graphene-silver hybrid suspension with microwave irradiation for a time period of 12 minutes to prepare irradiated TiO₂-graphene-silver hybrid suspension, wherein the irradiated TiO₂-graphene-silver hybrid suspension is dried at a temperature of 80° C. to prepare dried TiO₂-graphene-silver hybrid composite; and calcining the dried TiO₂-graphene-silver hybrid composite at a calcination temperature of 300° C. for a time period ranging from three to five hours to prepare the TiO₂-graphene-silver hybrid nanocomposite.
 2. The method of claim 1, wherein preparing the TiO₂-graphene-silver hybrid suspension includes reducing the suspension with 100 μl of hydrazine hydrate.
 3. The method of claim 1, wherein adding 0.5 M AgNO₃ solution to the TiO₂-graphene oxide sample includes selecting an amount of 0.5 M AgNO₃ from a group of 0.7, 1.77 and 3.55 ml.
 4. The method of claim 1 further includes washing the irradiated TiO₂-graphene-silver hybrid suspension with ethanol followed by distilled water.
 5. The method of claim 1, wherein calcining the dried TiO₂-graphene-silver hybrid composite includes increasing the calcination temperature at a rate of 1° C.-minute.
 6. A TiO₂-graphene-silver hybrid nanocomposite comprising: titanium oxide ranging from about 70 weight % to 98 weight %; graphene oxide ranging from about 0.1 weight % to 20 weight %; and silver ranging from about 2 weight % to 10 weight %, wherein the silver is formed on the titanium oxide; and wherein average particle size of the TiO₂-graphene-silver hybrid nanocomposite is ranging from 12-15 nanometers.
 7. The TiO₂-graphene-silver hybrid nanocomposite of claim 6, wherein the weight % of silver is selected from a group of 2%, 5% and 10%.
 8. The TiO₂-graphene-silver hybrid nanocomposite of claim 6, wherein titanium oxide is predominantly in an anatase phase.
 9. The TiO₂-graphene-silver hybrid nanocomposite of claim 6, wherein silver is predominantly in a cubic phase.
 10. A TiO₂-graphene-silver hybrid nanocomposite of claim 6 is employed in supercapacitors, assembled in a two-electrode system. 